The present invention relates to electronic signal amplifier circuits. In particular, the present invention is an audio frequency preamplifier that boosts the magnitude of signals obtained from an audio source, or instrumentation source in a similar frequency range, to form input signals for an audio frequency power amplifier that can be directly connected thereto without use of capacitors or coupling transformers. The present invention circuitry arrangement can also be used for a power amplifier that boosts the magnitude of audio frequency signals obtained from a preamplifier.
Preamplifier and power amplifiers for audio frequency signals are well known in the prior art. For example, U.S. Pat. No. 4,229,706 granted to Bongiorno in October 1980 and U.S. Pat. No. 4,719,431 granted to Karsten in January 1988 both disclose power amplifiers for this purpose. Some preamplifiers have been shown to be advantageous in being capable of transferring output signals therefrom over a balanced line interconnection arrangement directly connected thereto at its output. For example, the so called "Circlotron" circuit is such a high performance preamplifier.
An audio power amplifier 10 of the prior art, the "Circlotron" circuit, is illustrated in the FIG. 1. Power amplifier 10 includes first and second triode electron tubes 12 and 14 and first and second power supplies 16 and 18. Power amplifier 10 also includes first and second input terminals 20 and 22 as well as first and second output terminals 24 and 26. An output load 28 in operation is placed across the first and second output terminals 24 and 26. Finally, power amplifier 10 includes first and second stabilizing resistors 30 and 32 as well as first and second reference output resistors 34 and 36.
First and second power supplies 16 and 18 are constant polarity floating power supplies, that is, they do not have a ground reference with respect to the outputs thereof connected in power amplifier 10. Power supplies 16 and 18 include input terminals 17 and 19 suited for connection to ordinary 60 Hz single phase commercial alternating current (ac) sources, power transformers 21 and 23, full-wave bridge and rectifiers 25 and 27. Sources connected to terminals 17 and 19 provide a sinusoidal voltage waveform of substantially fixed amplitude to power amplifier 10. Power transformers 21 and 23 step down the voltage to an appropriate level for amplifier 10. Rectifying diode bridges 25 and 27 convert these ac waveforms to constant polarity waveforms between positive and negative output terminals having a selected nominal voltage value suited for operating the remainder of the circuit.
First triode 12 includes plate 40, grid 42, and cathode 44. Grid 42 is connected to input terminal 20 via resistor 30. Plate 40 is connected to the positive output terminal of first power supply 16. Cathode 44 is connected to first output terminal 24 and the negative output terminal of power supply 18. Second triode 14 includes plate 50, grid 52, and cathode 54. Grid 52 is connected to second input terminal 22 via resistor 32. Plate 50 is connected to the positive output terminal of second power supply 18. Cathode 54 is connected to second output terminal 26 and the negative output terminal of power supply 16. Resistor 34 is connected between first output terminal 24 and ground, and resistor 36 is connected between second output terminal 26 and ground. The cathode heater circuits for triodes 12 and 14 are not shown.
Power amplifier 10 typically receives from the input signal source in operation two balanced magnitude input voltage signals that are complements of each other, that is, one is the negative of the other ignoring the average values of each resulting from providing biasing for tubes 12 and 14. A first of these balanced input signals is received by first input terminal 20 and the second input signal is received by second input terminal 22. When the first input signal at terminal 20 is relatively high, the second input signal at terminal 22 is relatively low. Under these conditions, grid 42 of triode 12 has a relatively high voltage signal such that current increases through triode 12 to output terminal 24, and grid 52 of triode 14 has a relatively low voltage signal such that current decreases through tri ode 14 to output terminal 26 leaving a net voltage across the load. In this way, power amplifier 10 operates to amplify the difference between the first and second input signals to thereby provide a substantial differential current gain, though little voltage gain with tubes 12 and 14 each connected as cathode followers.
Audio power amplifier 10 has desirable characteristics such as wide frequency bandwidth, fast transient response and low total distortion for reasons set out below. Similarly, this design has desirable characteristics in preamplifier applications including wide frequency bandwidth, low total distortion, the ability to transfer output signals over relatively long interconnection cables, good reliability and low cost.
This design achieves wide frequency bandwidth because it has a relatively simple design that uses a small number of components. This relatively small number of components allows a circuit design with low parasitics. This design has fast transient response because, unlike other preamplifier designs, the relatively low distortion of this design allows avoiding the use of negative output signal feedback to correct distortion effects. Negative feedback introduces significant damping that inhibits transient response. Also, this design has low total distortion because the follower arrangement keeps the voltage across the load substantially in the linear portion of its characteristics, and since the signals are substantially balanced they cancel distortion effects in each other. Finally, this circuit can operate its load through relatively long interconnection cables because of its current gain and low output impedance.
However, there are still several problems to overcome even using prior art power amplifier 10. First, electrostatic noise from first and second power supplies 16 and 18 will result in noticeable amounts of noise at output terminals 24 and 26. Electrostatic noise is caused by the electrostatic coupling of the alternating current line voltage across the power transformers. Electrostatic noise tends to be a problem since even very small amounts of electrostatic coupling to the alternating current line voltage across from the power transformers can result in very noticeable amounts of noise at the output. Even where multiple electrostatic shields are used with the transformer, this typically does not eliminate all the problems. Furthermore, great care must be used in applying electrostatic shields to achieve even marginal improvement in amplifier noise reduction. The present invention greatly reduces or eliminates this problem.
In addition to this problem with power amplifier 10, average offset (DC) signal values of one polarity or the other occur at output terminals 24 and 26 due to circuit imbalances such as result from component parameter magnitude variations. Circuit imbalances may occur when there is a difference in gain between triodes that are not exactly matched or differences in voltage from the power supplies due to filter capacitor or resistor values being slightly off the proper value. The present invention also substantially overcomes these problems of the prior art.